1. Field of the Invention
The present invention relates to an optical apparatus having dust-off function, in which dust adhering to the inside of the optical apparatus can be removed, and more specifically, to an optical apparatus having a dust-off function for an image-pickup element such as an electronic image-pickup apparatus (digital camera or the like) and capable of also detecting an abnormal state of the dust-off function.
2. Description of Related Art
In an electronic image-pickup apparatus such as a digital camera, various mechanisms that mechanically operate are arranged in its apparatus body. Therefore, dust and the like that are generated from the mechanisms adhere to the photoelectric conversion surface of an image-pickup element, thereby inevitably degrading photographed images.
Accordingly, a technique is proposed as an example of a technique related to the dust-off function of an electronic image-pickup apparatus. According to this technique, dust or the like adhering to a protective glass plate (referred to as “dust-off glass” or “dust-off filter”) for protecting the image-pickup element is shaken off by vibrating the protective glass plate. In a prior art system, therefore, a piezoelectric element is provided as means for vibrating the protective glass plate, for example. The protective glass plate that is attached to the piezoelectric element is vibrated with a given period by utilizing the behavior of the piezoelectric element to extend or contract in response to cyclic voltage applied thereto.
According to the prior art described above, however, the amplitude of vibration of the dust-off protective glass plate itself is scanty.
In order to shake off dust securely, in general, it is advisable to maximize the amplitude of vibration of the glass plate. In order to increase the amplitude of vibration of the glass plate, however, the glass plate must be vibrated with its own natural resonance frequency. Otherwise, the glass plate can only vibrate with the displacement of the piezoelectric element itself, so that the dust cannot be shaken off efficiently.
Normally, the resonance frequency varies depending on the shape, material, supporting method, and vibration mode (vibration form) of the glass plate. If glass plates are mass-produced as protective glasses, moreover, the resonance frequency also varies owing to dispersion of working accuracy. However, the dispersion can be canceled by measuring the resonance frequency of each individual protective glass and suitably adjusting the frequency of an oscillator that applies voltage to the piezoelectric element during operation.
However, the aforesaid canceling method cannot cope with the dispersion if the resonance frequency of the protective glass drifts owing to aging and temperature change. Thus, even if the resonance frequency of the oscillator is securely adjusted, the protective glass cannot always be driven with its resonance frequency.
Accordingly, there is a demand for a system in which dust can be removed easily and efficiently without adjusting variations in factors (shape of the glass plate, modulus of elasticity of the material, etc.) that influence the resonance frequency of the glass plate for use as a protective glass during manufacturing processes for the protective glass or camera operation.
If a part of a dust-off mechanism goes wrong so that satisfactory vibration fails to be generated during operation, moreover, it may be anticipated that the situation will not be able to be recognized with the naked eye. Further, a measuring device such as a laser displacement gage is needed to determine whether or not the dust-off mechanism is abnormal.
However, general users cannot detect anything abnormal about the dust-off mechanism by means of the measuring device. The fact is that if a satisfactory dust-off effect cannot be obtained due to anything abnormal, the abnormality cannot be recognized.
The present invention has been contrived in consideration of these circumstances, and its first object is to provide an optical apparatus having dust-off function that includes a system such that a dust-off glass whose resonance frequency cannot be specified in one position can be driven efficiently, in an optical apparatus that has a dust-off function for shaking off dust or the like adhering to the dust-off glass on the front face of an image-pickup element, for example, by vibrating the dust-off glass. A second object is to provide an optical apparatus having dust-off function in which trouble in its dust-off function can be detected with ease.